30 31

C++返回對(duì)象和引用的區(qū)別

class A
{
punlic:
    A()
    {
        cout<<this<<" constructor"<<endl;
    }
    ~A()
    {
        cout<<this<<" destructor"<<endl;
    }
    A(const A & another)
    {
        cout<<this<<" cpy constructor from"<<&another<<endl;
    }   
    A& operator = (const A & another)
    {
        cout<<this<<" operator ="<<&another<<endl;
    }
};

//拷貝構(gòu)造發(fā)生的時(shí)機(jī)
//1.構(gòu)造新對(duì)象 A a ; A b = a;
//2.傳參或返回對(duì)象

//對(duì)于普通變量來說区转，傳引用效果不明顯
//對(duì)于類對(duì)象而言，傳對(duì)象效率很高
//傳引用等價(jià)于 擴(kuò)大了原對(duì)象的作用域

//棧上的對(duì)象是可以返回的胯盯，但不能返回棧上的引用(除非返回對(duì)象本身)
void func(A a)
{
    
}

int main()
{
    A x;
    A y = x;
    func(x);//發(fā)生一次拷貝構(gòu)造  函數(shù)不是引用的話 引用的話  就沒有拷貝構(gòu)造了
    return 0;
}

A func(A &a)
{
    return a;
}

int main()//一次構(gòu)造  一次拷貝構(gòu)造 
{
    A x;
    A t = func(x);
    cout<<"&t "<<&t<<endl;
    return 0;
}

int main()//兩次構(gòu)造 一次拷貝構(gòu)造 一次operator= 然后立馬析構(gòu)拷貝構(gòu)造的臨時(shí)變量
{
    A x;
    A t;
    t= func(x);
    cout<<"&t "<<&t<<endl;
    return 0;
}

A func()
{
    A b;
    return b;
}

int main()//兩次拷貝 一次operator = 然后立馬析構(gòu)b的
{
    A t;
    t= func();
    cout<<"&t "<<&t<<endl;
    return 0;
}

加法 大于 小于 等于 [] at

class myString
{
public:
    myString(const char *p = NULL);//默認(rèn)參數(shù)只能在聲明
    myString(const myString & another);
    myString& operator=(const myString & another);
    ~myString()车柠；
    
    myString operator+(const myString &another );
    bool operator>(const myString &another );
    bool operator<(const myString &another );
    bool operator==(const myString &another );
    char& operator[](int idx);
    char at(int idx);
   
   char *c_str();
private:
    char *_str;
}

myString myString::operator+(const myString &another)
{
    myString tmp;//char *=""; *this char * = "abcd",another char*="efg"
    delete []tmp._str;
    int len = strlen(this->_str);
    len += strlen(another._str);
    
    tmp._str = new char[len+1];
    memset(tmp._str,0,len+1);
    strcat(tmp._str,this->_str);
    strcat(tmp._str,another._str);
    return tmp;
}
bool myString::operator>(const myString &another )
{
    if(strcmp(this->_str,another._str)>0)
        return true;
    else
        return false;
}
bool myString::operator<(const myString &another )
{
    if(strcmp(this->_str,another._str)<0)
        return true;
    else
        return false;
}
bool myString::operator==(const myString &another )
{
    if(strcmp(this->_str,another._str)==0)
        return true;
    else
        return false;
}
char& myString::operator[](int idx)//xx[2] xx.operator[](int idx)
{
    return this->_str[idx];
}
char myString::at(int idx)
{
    return this->_str[idx];
}
int main()
{
    string x= "abc",y="efg";
    string z= x+y;
    cout<<z.c_str()<<endl;

    myString xx="abcd",yy ="efg";
    myString zz= xx +yy;//本質(zhì) xx.operator+(yy);
   cout<<xx[2]<<endl;
}